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Reaction Mechanism and Rate Law

Caleb Thomas

Caleb Thomas

7 min read

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Study Guide Overview

This study guide covers reaction mechanisms, including defining elementary steps, intermediates, and catalysts. It explains the rate-determining step and how to derive rate laws from mechanisms, including using equilibrium constants to account for intermediates. It also provides practice with free response and multiple-choice questions on these topics and includes a real AP Chem FRQ example.

Reaction Mechanisms & Rate Laws: Your AP Chem Survival Guide 🚀

Hey, future AP Chem rockstar! Let's break down reaction mechanisms and rate laws. This guide is designed to be your best friend the night before the exam—clear, concise, and super helpful. Let's do this!

1. Reaction Mechanisms: The Step-by-Step Story

What's a Mechanism? 🤔

  • A mechanism is like a detailed recipe for a reaction, showing exactly how reactants turn into products.
  • Most reactions happen in multiple steps, not just one big jump.
  • Think of it as the behind-the-scenes action of a chemical reaction.

Elementary Steps: The Building Blocks

  • Each step in a mechanism is an elementary step.
  • When you add up all the elementary steps, you get the overall balanced equation.
  • It's like combining all the individual ingredients to get the final dish.
Exam Tip

Remember: The sum of elementary steps = overall reaction.

Example Mechanism:

Mechanism Image

  • Overall Reaction: O₃ + 2I⁻ + H₂O → O₂ + I₂ + 2OH⁻
  • Elementary Steps:
    1. O₃ + I⁻ → O₂ + IO⁻ (slow)
    2. IO⁻ + H₂O → HOI + OH⁻
    3. HOI + I⁻ → I₂ + OH⁻

Catalysts and Intermediates: The Supporting Cast

  • Catalysts: Speed up reactions but aren't consumed (they're like a chef's favorite pan).
  • Intermediates: Formed and used up during the reaction (like a half-made dish).
Quick Fact

Catalysts are not part of the overall reaction, while intermediates are.

  • In the example above:
    • No catalysts are present.
    • Intermediates: IO⁻ (iodite) and HOI (hypoiodous acid).
Memory Aid

Think of Intermediates as being formed and then used up In the mechanism.

2. Rate-...

Question 1 of 11

Which of the following best describes an elementary step in a reaction mechanism? 🤔

A single-step reaction that represents the overall process

A multi-step reaction that shows the net change of reactants to products

One of the individual steps in a multi-step reaction mechanism

The slowest step in a reaction mechanism